1. Field of the Invention
The present invention is directed to ink jet printers, and in particular to ink jet printers having a flat, movable platen for receiving a recording medium (e.g. a sheet of paper) which maintains the recording medium flat as it is conveyed past an ink jet printhead. The present invention is also directed to such printers having an architecture that provides easy access between the printhead and a printhead maintenance station.
2. Description of Related Art
Ink jet printers can generally be divided into two types: one type using thermal energy to produce a vapor bubble in an ink filled channel that expels a drop of ink; or another type using a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle.
Thermal ink jet printers use thermal energy selectively produced by resistors located in capillary-filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printers can be incorporated in either a carriage-type printer or a pagewidth type printer. The carriage-type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and carriage assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. For an example of a carriage-type printer, refer to U.S. Pat. No. 4,571,599 to Rezanka, the disclosure of which is incorporated herein by reference. In contrast, the pagewidth printer includes a stationary printhead having a length equal to or greater than the width of the paper. The paper is continuously moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process. Refer to U.S. Pat. No. 4,463,359 to Ayata et al, the disclosure of which is incorporated herein by reference, for an example of a pagewidth printhead. Refer to U.S. Pat. No. 4,829,324 to Drake et al, the disclosure of which is incorporated herein by reference, for another example of a pagewidth printhead.
These thermal ink jet printheads are either of the side-shooter-type, having nozzles formed on a side of the printhead where two substrates are joined to each other, or of the roof-shooter-type having nozzles formed as apertures in an uppermost substrate (or "roof") of the printhead. The above-mentioned patents disclose side-shooter-type thermal ink jet printheads. For an example of a roof-shooter-type thermal ink jet printhead, see U.S. Pat. No. 4,789,425 to Drake et al, the disclosure of which is incorporated herein by reference.
Piezoelectric activated ink jet printing systems use a pulse generator which provides an electric signal. The signal is applied across crystal plates, one of which contracts and the other of which expands, thereby causing the plate assembly to deflect toward a pressure chamber. This causes a decrease in volume which imparts sufficient kinetic energy to the ink in the printhead nozzle so that one ink droplet is ejected into a recording medium. Refer to U.S. Pat. No. 4,144,537 to Kimura et al, the disclosure of which is incorporated herein by reference, for an example of a piezoelectric activated ink jet printer.
The present invention is applicable to printers employing thermal or piezoelectric activated printheads, as well as ink jet printheads relying on other types of ink droplet driving engines for controllably directing ink droplets onto a recording medium.
Conventional ink jet printers have an architecture wherein sheets are conveyed (either stepwise or continuously) past a printhead (either carriage-type or pagewidth) for having images printed thereon. The sheets are supported on a platen located closely adjacent to the printhead so as to maintain the sheet a precise distance spaced from the printhead nozzles. These platens either supply the motive force to the sheets to convey the sheets past the printhead, or merely act as a support. For example, the above-mentioned U.S. Pat. No. 4,463,359 to Ayata et al uses a cylindrical drum platen to convey a sheet past a printhead (see FIGS. 55-57). These cylindrical platens, however, can cause the sheets to curl, and can cause image distortion (or require nozzle-control compensation) since some of the printhead nozzles will be located further from the curved surface of the cylindrical platen than other nozzles.
U.S. Pat. No. Re. 32,572 to Hawkins et al discloses in FIG. 1, a printer architecture having a flat platen so that a sheet is everywhere equally spaced from the printhead nozzle. However, the platen does not move or convey the sheets past the printhead (a separate mechanism moves the sheets).
U.S. Pat. No. 4,207,578 to Marinoff discloses an ink jet printer where an endless belt conveys sheets past a printhead. The printhead can be arranged adjacent to a flat portion of the belt so as to maintain the sheet equally spaced from all nozzles of the printhead during printing.
These ink jet printheads usually require maintenance, for example, in order to: (1) clear clogged nozzles; (b) remove air from the printhead (air particularly interferes with droplet formation in thermal ink jet printheads); (c) clean dirt and excess ink from the nozzle-containing surface of the printhead; (d) cap the printhead nozzles during periods of non-use in order to prevent drying of ink in the nozzles; and (e) prime the printhead nozzles (individually, or all at once) at printer start-up, or even between sheets (inter-sheet purging).
Carriage-mounted printheads (where the printhead is relatively small and is mounted on a carriage that reciprocates across the width of a page) often include maintenance stations mounted at a side of the printing area, with the printhead being moved to the side of the printing area for having maintenance performed thereon. See, for example, U.S. Pat. No. 4,853,717 to Harmon et al. Since the printhead and platen must be located very close to each other, and it is not simple to provide an architecture where the platen and printhead are moved apart from each other so that a maintenance station can be moved therebetween, the location of the maintenance station alongside of the printing area has worked well with carriage-type printers.
However, in printers having pagewidth printheads, where the printhead extends entirely across a sheet, the printhead is stationarily mounted, and therefore the maintenance station can not be located to a side of the printing area. Pagewidth printheads can print much faster than carriage-type printheads, and therefore are preferred. However, thus far it has been difficult to integrate maintenance stations with existing pagewidth printhead architectures.
U.S. Pat. No. 5,051,761 to Fisher et al discloses a printer architecture wherein a maintenance station for a pagewidth ink jet printhead is located within an endless belt platen, or within a drum platen. The endless belt platen or drum platen includes a window through which the maintenance station is moved in order to engage the pagewidth printhead.
U.S. Pat. No. 4,207,578 to Marinoff discloses a carriage-type ink jet printhead having an endless-belt-type platen. As shown in FIGS. 3-4, the belt includes a slot which can be located across from the printhead for receiving ink and other materials expelled from the printhead during a flushing operation.
While both of these patents disclose architectures suitable for use with pagewidth printheads, any intersheet maintenance operations which might need to be performed (for example, cleaning and/or purging of some or all of the printhead nozzles) require the belt- or drum-type platens to be precisely located so that the window or slot is aligned with the printhead. Thus, even when the maintenance operation can be performed quickly, extra time may be required in order to precisely locate the platen slot relative to the printhead. Moreover, if such alignment is not precise, the platen may inadvertently receive ink, causing smudging of subsequent sheets.
U.S. Pat. No. 3,754,822 to Melrose discloses a scanning system for a copier which includes a moving transparent platen. A document to be copied is supported by the platen which is supported on a machine frame. The platen reciprocates across the frame past a scanner. An electrostatic plate is supported for movement along a plane parallel to the platen. The platen and the plate move to the right while the scanner moves to the left from an initial position to an end of scan position. The scanner and the platen then return to their respective initial positions while the plate continues to the right to a processing station. In another embodiment, shown in FIG. 4, a conveyor moves a paper sheet, to receive an image from the document from a supply tray past the scanner.
U.S. Pat. No. 3,737,223 to Yamamoto discloses an apparatus for driving a platen in an electrophotographic copying machine. The apparatus includes a platen, a drive roller in frictional contact with a bottom of the platen, a platen support means, a power means for driving the drive roller to move the platen along a horizontal plane, and depressing means for depressing the platen onto the drive roller to prevent vertical movement of the platen. The apparatus is used to convey an original document located on the platen from an initial position, past a scanner, and reciprocate the platen, including the original document, back to the initial position.
Although U.S. Pat. Nos. 3,754,822 and 3,737,223 disclose reciprocating platens, these platens are not used to convey blank sheets past a printing mechanism. Additionally, these patents do not suggest the use of reciprocating platens for conveying sheets past an ink jet printhead, or the advantages obtained thereby.